Apparatus and method for handling exception events

Abstract

Processing circuitry 4 has a plurality of exception states EL0-EL3 for handling exception events, the exception states including a base level exception state EL0 and at least one further level exception state EL1-EL3. Each exception state has a corresponding stack pointer indicating the location within the memory of a corresponding stack data store 35. When the processing circuitry is in the base level exception state EL0, stack pointer selection circuitry 40 selects the base level stack pointer as a current stack pointer indicating a current stack data store for use by the processing circuitry 4. When the processing circuitry 4 is a further exception state, the stack pointer selection circuitry 40 selects either the base level stack pointer or the further level stack pointer corresponding to the current further level exception state as a current stack pointer.

Description

[0001]This application is a Continuation of U.S. patent application Ser. No. 13 / 064,108, filed Mar. 7, 2011, which claims priority to GB Application No. 1004303.2 filed 15 Mar. 2010. The entire contents of these applications are incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to the field of data processing. More particularly, the invention relates to an apparatus and method for handling exception events.[0004]2. Description of the Prior Art[0005]While data processing is being carried out, an exception event may occur, indicating that the data processing should be interrupted so that an exception handling routine can be performed. Exception events can indicate, for example, that a processing result is undefined, that a processing resource has overflowed, or that an exception signal has been received from an external device. It is possible to process exception events in multiple exception states. Each exce...

AI Technical Summary

Benefits of technology

The patent describes an apparatus for handling exception events in a computer program. The apparatus has multiple stack data stores for storing temporary variables and performs processing based on the status of the program. By using a base level stack data store for temporary variables and a further level stack data store for more general processing, the apparatus reduces the risk of stack overflow and ensures correct handling of exception events. The apparatus also provides selection circuitry to select between the two stack pointers and control the processing circuitry to use the currently selected stack pointer to access one of the stack data stores. Overall, the apparatus improves the efficiency and safety of exception handling in computer programs.

Problems solved by technology

If too much data is stored on the stack, then the stack may exceed its allocated memory space.

The stack may overrun a page boundary and may not have the access rights for accessing the next page in memory, thus triggering a memory abort.

Also, if the processor tries to store on the stack a variable that is larger than the remaining space in the stack, then this can cause an error.

If the stack overflows, then this could prevent the exception handling routine being executed properly, since the stack overflow may itself trigger an exception event which causes another exception handling routine to try to push variables onto the stack, causing another stack overflow exception event.

This can result in an endless loop of exception events each trying to load variables onto the stack and each triggering a further exception event due to the stack overflow, and can cause the system to crash due to such recursive exception entry.

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